A known phenomenon in rectified electrical systems is the harmonics which are introduced by the rectification units. Typically, for a six pulse system, these harmonics are significant in the 5th and 7th order. Such harmonics can result in the associated electrical equipment being effectively over rated in relation to the electrical power required by the supplied loads.
FIG. 1 shows an electrical system 10 which seeks to reduce the effect of the 5th and 7th harmonics by providing a so-called dual lane twelve pulse power supply for an electrical drive. Each of the lanes 12, 14 includes a generator 16a, 16b, a transformer 18a, 18b and a twelve pulse rectifier unit 20a, 20b, which includes two six diode bridges 22a, 22b. The twelve pulse rectifier provides DC power to a load 24a, 24b. The load 24a, 24b can be any which requires DC power, such as an electric motor or a convertor for supplying power to an AC (or DC) load. The transformer 18a, 18b is of the star-delta, YD, type and is used to provide a thirty (30) degree phase difference in the output branches which allows a reduction in the harmonic content of output from the rectifier unit 20a, 20b. The use of YD arrangements to provide a thirty degree phase shift is well known.
FIG. 2 shows an alternative electrical system 210 in the form of an electrical propulsion arrangement in which power is provided to two electrical propulsive motors 224a, 224b via a convertor 228a, 228b. Each convertor 228a, 228b receives DC power from a six pulse rectifier 220a, 220b which is connected to an AC supply in the form of a generator 216a, 216b. The two fully rated three winding transformers 18a, 18b shown in FIG. 1 have been removed in the system of FIG. 2 and replaced with a two winding star-delta, YD, transformer 218 which is located between and electrical couples the two buses 226a, 226b of the two lanes 212, 214. With equal loading of each electrical motor 224a, 224b, the YD connected transformer 218 provides a path for the circulation of the dominant 5th and 7th harmonic currents generated by each diode bridge rectifier 220a 220b and prevents these undesirable currents from flowing in each of the generators 216a, 216b. Given these harmonic currents are typically 30% of the main fundamental frequency currents, the bus coupling transformer 218 can be of a substantially lower rating in comparison to the three winding transformers shown in FIG. 1.
A further known improvement to the described scheme is a tertiary winding (not shown) to provide a YYD transformer which incorporates a high frequency harmonic filter. Such a filter can further improve the generator current by reducing 11th and 13th harmonic current components.
U.S. Pat. No. 7,514,898 provides an example of a known YD connected transformer for filtering 5th and 7th harmonic currents.
The present invention seeks to provide an improved electrical arrangement for providing rectified electrical power.